As is well known, methane is the largest component of natural gas, and usually accounts for at least 95% by volume of what is known as "transmission specification" natural gas. Other usual components are ethane (usually about 2%), propane (usually about 0.5%), butanes, pentanes and possibly hexanes (altogether amounting to less than about 0.3%), with the balance being nitrogen and carbon dioxide. In this disclosure, transmission specification natural gas will be hereinafter called "natural gas". For example, the natural gas as transmitted through the pipelines of TransCanada Pipeline Limited from Alberta, Canada to Ontario, Canada has typically the following percentage composition by volume:
Component Feed Nitrogen 0.01270 Carbon Dioxide 0.00550 Methane 0.95400 Ethane 0.01970 Propane 0.00510 i-Butane 0.00170 n-Butane 0.00080 i-Pentane 0.00020 n-Pentane 0.00010 n-Hexane 0.00020
The relation between pressure, volume and temperature of a gas can be expressed by the Ideal Gas Law, which is stated as PV=nRT, where:
P=pressure of gas PA1 V=volume of gas PA1 n=number of moles of the gas PA1 R=the universal gas constant (which, as is known, varies somewhat depending on volume and temperature) PA1 T=temperature of the gas. PA1 T=the temperature of the gas in degrees R PA1 T.sub.c =the critical temperature of the gas in degrees R PA1 P=the pressure of the gas in psia PA1 P.sub.c =the critical pressure of the gas in psia
If the equation is expressed in English units, the pressure is in pounds per square inch absolute (psia), the volume is in cubic feet, and temperature is in degrees R (degrees Fahrenheit plus 460).
The Ideal Gas Equation does not give exactly correct results in actual practice, because gases are compressible. Gas molecules, when compressed, pack more tightly together than would be predicted by the Ideal Gas Equation, because of intermolecular forces and molecular shape. To correct for this, an added term, the compressibility factor z, can be added to the Ideal Gas Equation. This is a dimensionless factor which reflects the compressibility of the particular gas being measured, at the particular temperature and pressure conditions.
At atmospheric pressure or gage pressures of a few hundred pounds, the compressibility factor is sufficiently close to 1.0 so that it can be ignored for most gases, and so that the Ideal Gas Law can be used without the added term z. However, where pressures of more than a few hundred pounds exist, the z term can be different enough from 1.0 so that it must be included in order for the Ideal Gas Equation to give correct results.
According to the van der Waals theorem, the deviation of a natural gas from the Ideal Gas Law depends on how far the gas is from its critical temperature and critical pressure. Thus, the terms T.sub.R and P.sub.R (known as reduced temperature and reduced pressure respectively) have been defined, where ##EQU1##
where,
Critical pressures and critical temperatures for pure gases have been calculated, and are available in most handbooks. Where a mixture of gases of known composition is available, a pseudo critical temperature and pseudo critical pressure which apply to the mixture can be obtained by using the averages of the critical temperatures and critical pressures of the pure gases in the mixture, weighted according the percentage of each pure gas present.
Once a pseudo reduced temperature and pseudo reduced pressure are known, the compressibility factor z can be found by use of standard charts. One of these is "Compressibility Factors for Natural Gases" by M. D. Standing and D. L. Katz, published in the Engineering Data Book, Gas Processors Suppliers Association, 10th edition (Tulsa, Okla. U.S.A.) 1987.
When the compressibility factor z of methane is read from the charts, it is found that the factor z is always less than 1.0 in normal temperature ranges (i.e. between about -40.degree. F. and 120.degree. F.) and that it decreases as the pressure rises or the temperature falls. Therefore, less energy need be used to pump a given volume of methane (measured at standard volume) at any given normal temperature than would be expected at that temperature if the methane were an ideal gas. This effect is more marked at higher pressures. Similarly, as the pressure is increased at a constant temperature, more methane (measured at standard volume) can be stored in a given volume than would be predicted from the Ideal Gas Equation. "Standard volume" is volume measured at standard pressure and temperature (STP)).
Natural gas, like methane, shows z factor changes with pressure. Under about 1000 psia the dominant variable in the power relationship is the molecular weight of the gas. At this pressure level, addition of further amounts of ethane or propane increases the molecular weight of the gas more rapidly than the z factor decreases. Thus, there is an advantage to removing ethane and propane from the gas.
It is usual in the gas transportation and storage industry to try to strip out higher hydrocarbons such as ethane, propane, butane and unsaturated hydrocarbons from natural gas if the gas is to be transmitted through pipelines. This leaves mostly methane (with some traces of nitrogen and carbon dioxide) to be transported by the gas pipeline. The materials which are stripped out are then transported or stored separately, often as liquids.